Consumption patterns of the preserved vegetable, a staple dish of migrant workers, helped researchers track labourers’ movement within China, an unnamed staffer of the planning department of the National Development and Reform Commission (NDRC) told the Economic Observer.

The NDRC is expected to soon unveil reforms that would allow migrant workers to enjoy some social services in cities where they do not hold a local hukou, or household registration, in an effort to boost consumption. The reforms are, however, expected to put considerable financial burden on cities with a heavy presence of migrant workers.

One in five Chinese was a migrant worker at the end of last year, according to the National Bureau of Statistics. And 62 per cent of them work outside their home provinces. But the trend is shifting, and growth rates are slowing.

The number of rural Chinese who left their villages to see work outside their home provinces increased only 3.9 per cent last year, compared with 2011.The growth rate stood at 4.4 per cent in 2011 and at 5.4 per cent in 2010.

The number of migrant workers in the Pearl River Delta decreased by 0.5 per cent last year and by 0.3 per cent in the Yangtse River Delta.

Sales of the pickled plant, or zhacai, from the Chongqing district of Fuling, appear to support the slowing trend. The figures show how many workers have moved back from southern China’s economic power hubs to their home provinces inland over the last years.

The nation’s largest producer of the sour-spicy vegetable, Chongqing Fuling Zhacai, saw its share of sales in southern China decrease drastically as sales in central China increased.

Southern China's share in the company's sales fell from 49 per cent in 2007 to 30 per cent in 2011. The share of the central provinces Henan, Hubei and Hunan, increased from 2.6 per cent in 2009 to 10.6 per cent last year, and in other inland provinces by around 2 percentage points.

Chongqing Fuling Zhacai, saw revenue from inland provinces increase between 45 per cent and 57 per cent in 2011, compared with a year earlier, the newspaper said. Revenue in the southern provinces increased by only 1.3 per cent.

This article appeared in the South China Morning Post print edition as How pickle sales can help track migrants